But why many DT don't tag? Can you please share your opinion why don't you?
Scammers target mostly inexperienced people who have no idea what trust is or what flag means, so you shouldn't think that there would be anything better on the forum in that regard if all scammers were taged&flaged. In addition, no scammer will be prevented from continuing to do what he did before someone left him negative feedback.
The question of why more DT members do not use their powers can be put in the same context as the question of why more forum members do not use the "report to moderator" button? I know the answer to the second question, and I think it can be applied to the first question as well - probably because all free work is considered a waste of time.